1. Field of the Invention
The present invention relates to a stage system, a lithographic apparatus including such stage system.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In recent developments in lithography, use is made of a stage system (including, e.g., a wafer stage or a mask stage), which is provided with an encoder system to measure a position of a movable part of the stage, such as the substrate support (e.g., wafer table) or patterning device support (e.g., mask table). Thereby, use may be made of encoder targets, such as grating plates, grid plates, etc. In some implementations, use may be made of a plurality of such encoder targets which are positioned adjacent to each other in order to, e.g., provide a larger measurement range as compared to a measurement range that would have been obtained with a single encoder target. Many considerations may result in such arrangements: it may for example be the case that production constraints limit a maximum achievable size of such encoder target, while a desired range of movement of the stage, and hence a desired measurement range, would exceed such size of the encoder target. Furthermore, it would be possible that the encoder targets would need to be provided around an element, such as a projection system, which would invoke a need to apply a plurality of encoder targets in order to surround the element.
When making use of a plurality of encoder targets, irregularities in the position measurement may occur when the stage moves from cooperating with one encoder target to cooperating with another one of the encoder targets. Thereby, a discontinuity, a step, an overlap or other effect may occur which may result in an inaccuracy of a position, movement, etc of the stage.
As an example, a transition from one encoder target to another one, each of the encoder targets having its own, associated error, may, when moving the stage from an area where sensor(s) of the stage cooperate with one encoder target, to an area where the sensors cooperate with the other encoder target, result in a disturbance of such stage movement as the position of the stage is determined first with reference to the one encoder target and then to the other encoder target.